Modify encryption callback to expect a binary string in Python API (openvinotoolkit#28953)

### Details:
- Cache encryption callbacks in GPU Plugin operate on strings that
contain binary data which cannot be converted into a valid Python string
(C++ strings are encoding-agnostic while Python strings are Unicode).
This change modifies encryption callbacks to operate on binary strings
(`bytes` type) on Python side.

Author: tkrupa-intel

docs/articles_en/assets/snippets/ov_caching.py

@@ -56,11 +56,11 @@
 # ! [ov:caching:part5]
 import base64
 
-def encrypt_base64(src):
-    return base64.b64encode(bytes(src, "utf-8"))
+def encrypt_base64(src: bytes):
+    return base64.b64encode(src)
 
-def decrypt_base64(src):
-    return base64.b64decode(bytes(src, "utf-8"))
+def decrypt_base64(src: bytes):
+    return base64.b64decode(src)
 
 core = ov.Core()
 core.set_property({props.cache_dir: path_to_cache_dir})
@@ -73,14 +73,14 @@ def decrypt_base64(src):
 # ! [ov:caching:part6]
 import base64
 
-def encrypt_base64(src):
-    return base64.b64encode(bytes(src, "utf-8"))
+def encrypt_base64(src: bytes):
+    return base64.b64encode(src)
 
-def decrypt_base64(src):
-    return base64.b64decode(bytes(src, "utf-8"))
+def decrypt_base64(src: bytes):
+    return base64.b64decode(src)
 
 core = ov.Core()
-if "GPU" in core.available_devices:
+if any("GPU" in device for device in core.available_devices):
     core.set_property({props.cache_dir: path_to_cache_dir})
     config_cache = {}
     config_cache["CACHE_ENCRYPTION_CALLBACKS"] = [encrypt_base64, decrypt_base64]

src/bindings/python/src/pyopenvino/utils/utils.cpp

@@ -298,14 +298,14 @@ std::map<std::string, ov::Any> properties_to_any_map(const std::map<std::string,
                 [py_encrypt](const std::string& in_str) -> std::string {
                 // Acquire GIL, execute Python function
                 py::gil_scoped_acquire acquire;
-                return (*py_encrypt)(in_str).cast<std::string>();
+                return (*py_encrypt)(py::bytes(in_str)).cast<std::string>();
             };
 
             std::function<std::string(const std::string&)> decrypt_func =
                 [py_decrypt](const std::string& in_str) -> std::string {
                 // Acquire GIL, execute Python function
                 py::gil_scoped_acquire acquire;
-                return (*py_decrypt)(in_str).cast<std::string>();
+                return (*py_decrypt)(py::bytes(in_str)).cast<std::string>();
             };
             ov::EncryptionCallbacks encryption_callbacks{encrypt_func, decrypt_func};
             properties_to_cpp[property.first] = encryption_callbacks;

src/bindings/python/tests/utils/helpers.py

@@ -198,11 +198,11 @@ def generate_relu_compiled_model(
 
 
 def encrypt_base64(src):
-    return base64.b64encode(bytes(src, "utf-8"))
+    return base64.b64encode(src)
 
 
 def decrypt_base64(src):
-    return base64.b64decode(bytes(src, "utf-8"))
+    return base64.b64decode(src)
 
 
 def generate_relu_compiled_model_with_config(